Stealth And Detection In Hills And Mountains
As a guideline, the maximum distance in mountain terrain at which a creature can succeed at a Perception check to detect the presence of others is 4d10×10 feet. In hill terrain, the maximum distance is 2d10×10 feet. It’s easier to hear distant sounds in the mountains. The DCs of Perception checks that rely on sound are usually lower at the GM’s discretion.
Source
Core Rulebook pg. 397
